“This was more of an execution: walk in with the specific mindset to shoot police officers,” he said.

The attack, which happened at 8.15am, was clearly targeted at the officers, not a robbery gone bad, according to Mr Troyer. “There were marked patrol cars outside and they were all in uniform.”

The Forza Coffee Shop is one of a popular local chain, on a side street in Tacoma, south of Seattle. The shop is in a small retail centre alongside two restaurants, a cigar store and a nail salon.

Investigators were immediately considering whether the incident was a copycat killing following the murder one month ago of a Seattle police officer. Timothy Brenton was shot and killed on Halloween night as he was sitting in a patrol car with a trainee, Britt Sweeney, who was grazed in the attack. Mr Brenton's suspected killer Christopher Monfort (41) remains in hospital after a shoot-out two days later.

Brad Carpenter, founder and owner of Forza Coffee, said his staff were being interviewed by police. He said his main concern was with the families of the police officers. “I'm a retired police officer, so this really hits close to home for me,” he told reporters.

Streets around the coffee shop were blocked off yesterday and a police helicopter hovered over a large crowd of investigators. Police were seen taking away a truck which had been parked outside a grocery store.

“We are looking at some people, we are looking at some cars, we are looking at some residences,” Mr Troyer said.

Washington Governor Chris Gregoire said she was “shocked and horrified” by the killings. “Our police put their lives on the line every day and tragedies like this remind us of the risks they continually take to keep our communities safe. My heart goes out to the family, friends and co-workers of these officers, as well as the law enforcement community.”

Police said they had not ruled out the theory that a second person may have aided the killer.